Diamond heat sink

ABSTRACT

A heat sink made of a natural or polycrystalline diamond substrate with fins formed thereon. Diamond is grown to form a substrate and a laser is used to cut channels in the substrate to form the fins.

RELATED APPLICATIONS AND PRIORITY CLAIM

This application is a divisional application of prior U.S. patentapplication Ser. No. 10/114,601 filed on Apr. 2, 2002, which is herebyincorporated herein by reference, and to which this application claimspriority.

FIELD OF THE INVENTION

This invention relates to a novel heat sink made of natural orpolycrystalline diamond.

BACKGROUND OF THE INVENTION

State of the art cooling systems with integral natural orpolycrystalline diamond heat spreaders include a heat source such as aRF power amplifier chip, a diode chip or chip array that may be lightemitting, or a power regulator chip attached to a diamond submount,which serves as a heat spreader. The thermal purpose of the diamond heatspreader is to reduce the intensity level of the heat flux emanatingfrom the heat source, thereby making it more amenable to transfer tomore conventional heat sink materials such as copper or aluminum whichpossess poorer thermal transport properties than diamond. Copper oraluminum materials are formed into heat sinks for the purpose of furtherreducing the heat flux density thereby allowing its efficientintroduction into the heat rejection medium which might be gaseous orliquid, or even a solid thermal mass. Thus, heat dissipated in theelectrical component flows through a complex mechanical assemblyencountering several interfaces along the way.

Unfortunately, each interface resists heat flow which must be overcomeby increasing the temperature in the assembly and ultimately, at thesource. Special precautions are taken at each interface in order toreduce the resistance to heat flow. The bottom of the heat source andthe diamond heat spreader are plated with special materials that enhancetheir affinity to low resistance interface materials including solderssuch as gold/tin eutectic. However, high temperatures persist at thesource leading to premature electrical failure of the power-dissipatingdevice and causing system failure and downtime and increasingsystem-operating expense. Alternatively, complex and bulky refrigerationsystems are required to lower device temperatures to acceptable levels.Frequently, these systems are incapable of dramatically reducing devicetemperature.

For example, one arrangement consists of an RF power amplifier chipsoldered to a diamond submount, which in turn is soldered to chipcarrier made of copper molybdenum. The carrier is adhesively bonded toan amplifier package, which in turn is bonded to an aluminum heat sink.A refrigerated anti-freeze solution flows over the fin-like surfaces ofthe heat sink picking up the dissipated heat and carrying it away fromthe heat source for ultimate rejection to the environment.

As solid state electrical devices are made smaller and smaller and yetat the same time designed to process more power and thus more heat,researchers are continuously looking for ways to lower the thermalresistance for heat transfer from the active regions of the device tothe environment.

In response, those skilled in the art have attempted to etchmicrochannels in the base of silicon devices and to mount laser diodearrays on silicon in which the microchannels have been etched. See U.S.Pat. No. 5,548,605. Another approach uses epitaxial lift-off (ELO) andgrafting which yield epitaxial GaAs films of thickness as thin as 200 Åon diamond substrates. See Goodson et al., “Improved Heat Sinking forLaser-Diode Arrays using Microchannels in CVD Diamond”, IEEETransactions on Components, Packaging, and Manufacturing Technology—PartB, vol. 20, No. 1, February 1997, incorporated herein by this reference.

In this article, the authors theorized that microchannels could beformed in diamond instead of silicon to lower the thermal boundaryresistance since diamond is the best heat conductor known. The idea offorming microchannels in diamond, however, was only notional and theauthors provided only a theoretical basis for unexplained futureexperimental work: “future experimental work needs to include severaltechnological innovations that make the proposed cooling system readyfor practical implementations.” Id. page 108 (emphasis added).

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a heat sink madeof natural or, more typically, polycrystalline diamond suitable forpractical implementations.

It is a further object of this invention to provide such a heat sinkwhich greatly reduces resistance to heat transfer from a heat sourcesuch as a power amplifier chip or a laser diode array to theenvironment.

It is a further object of this invention to provide such a heat sinkwhich eliminates many of the interfaces between the power dissipatingdevice and the environment.

This invention results from the realization that the thermal resistanceto heat transfer from a heat source such as a power amplifier chip or asemiconductor laser-diode array to the environment can be improved andnumerous interfaces between the power dissipating chip and the heat sinkeliminated by using a laser to cut microchannels in the diamond submountpreviously used as a lateral heat spreader thereby converting thediamond submount into a heat sink with the remaining diamond materialacting as heat transfer surfaces or fins and defining microchannelsbetween the fins.

This invention features a heat sink comprising a natural orpolycrystalline diamond substrate with fins formed preferably via lasercutting operations thereon. In the preferred embodiment, the diamond ischemical-vapor deposited diamond but it may also be diamond-like-carbon.The fins typically extend continuously along the substrate but mayinstead be pin fins. In some embodiments, the substrate and the fins aremonolithic. In other embodiments, substrate has a plurality of layersand the fins are cut in all of the layers or instead only a subset ofall the layers.

In the preferred embodiment, the fins form microchannels in thesubstrate. Typically, the substrate has opposing top and bottom planarsurfaces and the fins are formed in one said planar surface. In otherembodiments, however, the fins are formed in an edge of the substrate.

An integrated cooling system in accordance with the subject inventionincludes a heat source, a heat sink made of a natural or syntheticdiamond substrate with fins formed thereon mounted to the heat source, ametalization layer at the interface between the heat source and the heatsink, and a bonding layer between the metalization layer and the heatsource for securing the heat source to the heat sink. In the preferredembodiment, the metalization layer is a gold layer formed on the heatsink substrate and there is also a metalization layer formed on the heatsource and mated with the bonding layer. The bonding layer is typicallysolder, but may also be braze, or formed by compression bonding.

An optical device in accordance with the subject invention includes areflective surface and a heat sink adjacent the optical surface, theheat sink including a natural or polycrystalline diamond substrate withfins formed thereon.

A window in accordance with this invention includes a natural or apolycrystalline diamond substrate with upper and lower surfaces and finsformed in at least one edge of the substrate.

A method of manufacturing a diamond heat sink according to thisinvention includes growing diamond to form a substrate and using a laserto cut channels in the substrate to form fins thereon. In someembodiments, multiple diamond plates are grown and secured together toform a substrate with discrete layers before the channels are cut. Thechannels may be cut in all the layers or only in a subset of the layers.

Chemical-vapor-deposition of diamond is the preferred technique forgrowing the diamond and the channels are preferably cut to be 150 um orless in width to form microchannels.

The channels may be cut to extend in one direction to form straight finsor instead cut to extend in two different directions to form pin fins. Ametalization layer may be added to the substrate and substrate polishedbefore it is cut by the laser.

In another embodiment, the heat source is mounted to a diamond supportor strong back which is attached to the diamond heat sink.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a prior state-of-the-artintegrated cooling system using microchannel flow passages in analuminum heat sink;

FIG. 2 is a schematic cross-sectional view of an integrated coolingsystem incorporating a diamond heat sink in accordance with the subjectinvention;

FIG. 3 is a schematic view of a rectangular cross-section pin finembodiment of the diamond heat sink of the subject invention;

FIG. 4 is a top view of a parallelogram cross-section pin fin embodimentof the diamond heat sink of the subject invention;

FIG. 5 is a photograph of a diamond heat sink with deepcut microchannelsmanufactured in accordance with the subject invention;

FIG. 6 is a photograph of a diamond heat sink with shallow cutmicrochannels manufactured in accordance with the subject invention;

FIG. 7 is a schematic cross sectional view of a diamond structuralstrongback embodiment of the subject invention;

FIG. 8 is a schematic cross sectional view of a laminated diamondstructural strongback embodiment of the subject invention;

FIG. 9 is a photograph of an integrated assembly manufactured inaccordance with the subject invention;

FIG. 10 is a schematic view of an edge cooled electromagnetic or lightwindow embodiment of the diamond heat sink of the subject invention;

FIG. 11 is a schematic top view of a diamond optical or electromagneticenergy reflector with integral cooling channels in accordance with thesubject invention;

FIG. 12 is a bottom view of the reflective device shown in FIG. 11; and

FIG. 13 is a schematic view of a RF power amplifier mounted in a radarembodiment of the diamond heat sink in accordance with the subjectinvention.

DETAILED DESCRIPTION OF THE INVENTION

As delineated in the Background of the Invention section above, a priorstate of the art in integrated cooling systems is shown in FIG. 1. Heatsource 10 is an electrical device which may be a power amplifier chip ora laser diode array with a gold adherence promoting metalization layer12 plated on the bottom side thereof as shown. A heat spreader in theform of diamond submount 18 is metalized on both sides with adherent 16and 20. AuSn solder layer 14 secures heat source 10 to diamond submount18. Another AuSn solder layer 22 secures this subassembly to coppermolybdenum carrier 24. Solder layer 26 is used to secure carrier 24 toaluminum silicon carbide (AlSiC) package 28. Package 28 is adhesivebonded as shown by layer 30 to aluminum microchannel heat sink 36.Antifreeze coolant flows between fins 32 in channels 34 to remove heatemanating at source 10.

In one embodiment, heat source 10 is a power amplifier chip 6.6 mm by4.9 mm in area including a GaN layer 2 um thick and a SiC layer 100 umthick. Gold adherent layers 12 and 16 are 5 um thick and AuSn solderlayer 14 is 5 um thick. Diamond submount 18 is 380 um thick. Goldadherent layer 20 is 5 um thick and AuSn solder layer 22 is 5 um thick.CuMo carrier 24 is 1 mm thick and approximately 25 mm by 25 mm in area.Solder layer 26 is 50 um thick. AlSiC package 28 is 1 mm thick. Adhesivelayer 30 is 250 um thick. Heat sink 36 comprises a 1 mm face plate witha fin pitch of 0.32 mm, and 150 um thick aluminum fin stock 2 mm high.Fins 32 interface with a coolant such as an ethylene glycol/watercomposition at a 20° C. inlet temperature. When the coolant which flowsthrough the microchannels 34 (e.g., 150 um or less in width) betweenfins 32 is at this temperature, the temperature of the active regions ofthe power amplifier chip 10 (determined by computer modeling) can bemaintained at 233° C.

Still, as devices are made smaller and smaller and yet at the same timeprocess more power and thus generate more heat, researchers havesearched for ways to lower the resistance to heat transfer from theactive regions of device 10 to the environment as represented by thecoolant flowing between the microchannels 34 of heat sink 36.

This invention results in part from the realization that if diamondsubmount 18 is made thicker and is cut with a laser to formmicrochannels, it will then perform two functions: lateral spreading ofheat from the active regions of chip 10 and the heat transfer functionpreviously supplied by aluminum heat sink 36. Thus, aluminum heat sink36 can be eliminated and at the same time many undesirable interfaceswhich impede heat transfer are also eliminated (e.g., solder layer 22,carrier 24, solder layer 26, and adhesive layer 30). In addition,manufacturing process steps are eliminated including two solderingsteps, one adhesive bonding operation and two gold plating steps.

Accordingly, the subject invention features diamond heat sink 40, FIG.2. Heat sink 40 is made by cutting channels 50 in diamond submount 52 toform fins 54. Submount 52 is typically polycrystallinechemical-vapor-deposition (CVD) diamond but could also bediamond-like-carbon or even natural diamond. Fins 54 typically extendcontinuously along one surface (typically the bottom) of the diamondsubmount but if channels 50 are cut in two directions, pin fins 53, FIG.3 may be formed. In FIG. 4, pin fins 53′ have a parallelogramcross-section.

As shown in FIG. 2, the top surface of heat sink 40 typically includesgold adherent plated layer 16 which is attached via AuSn solder layer 14to gold adherent layer 12 of heat source 10. Other types of solder orbrazing materials or even compression bonding techniques may be used tosecure heat sink 40 to heat source 10.

A comparison of FIGS. 1 and 2 reveals the advantages of the diamond heatsink of the subject invention: in FIG. 2, the following components ofFIG. 1 are eliminated: adherent 20, solder 22, carrier 24, adhesive 26,AlSic package 28, SnPb solder layer 30, and aluminum heat sink 36. Inaddition, heat sink 40, FIG. 2 performs the function of diamond submount18, FIG. 1 (lateral heat spreading) and yet also acts as the coolinginterface by virtue of fins 54, FIG. 2 and channels 50. Furthermore, inFIG. 1, when the coolant was at 20° C., the maximum temperature of heatsource 10 was 233° C. In contrast, in accordance with the design of FIG.2, when the coolant was at 20° C., the maximum temperature of heatsource 10 (determined by computer modeling) was much lower—162° C. Assuch, novel diamond heat sink 40 can result in the elimination of therefrigeration subsystems of the prior art for a given devicetemperature, while reducing the temperature of the heat source thusimproving electrical performance and increasing the useful life of theheat source. The elimination of the refrigeration system reduces systemweight, space, power consumption and maintenance requirements.

The microchannels cut in the diamond submount may be forced to intersectone another if the laser cutter is so programmed. FIG. 3 shows arectangular cross-section ‘pin-fins’ 53 that results from channels cutalong orthogonal axes. The microchannels may also be formed to have aparallelogram cross-section by cutting along non-orthogonal axes, asshown in FIG. 4. These channels forming approaches may be beneficial toreduce coolant hydrodynamic boundary layer build-up along the coolantflow through the microchannels, and thus enhance heat transfer, orreduce coolant pressure drop. In either case, the basic stack-up of FIG.2 is maintained, with the power dissipating device 10 attached via goldadherent 12 to solder 14 and another gold adherent layer 16 on the heatsink 40.

As shown in FIGS. 5 and 6, microchannels 50 cut by a laser in a CVDsubmount 52 may be 150 um in width or less, may be 0.5 mm (FIG. 6) to0.8 mm (FIG. 5) in depth or greater, and may have walls contoured tocorrespond to the reduced fin cross sectional area required to conductheat at fin tips. The latter may reduce coolant pressure drop andsubsequent pumping power requirements.

FIG. 7 is a schematic representation of diamond microchannel heat sink63, which has been attached to a diamond structural support orstrongback 66. In this embodiment, the power dissipating device 10 isplated with gold adherent 12 and attached to diamond strongback 66 viaAuSn solder 14. Strongback 66 has been plated top and bottom with goldadherent 16, thereby allowing the heat sink 63 to be soldered tostrongback 66. Strongback 66 provides mechanical support for heat sink63 and the electrical devices as well as providing an electrical groundplane for those devices. In addition, the strongback serves as a carrierfor the electrical devices as well as providing a mounting frame forbonding into higher assemblies. Another feature of strongback 66 is thatit can emulate the heat spreader function of the current diamond and, assuch, can be made of higher quality diamond than heat sink 63, which canreduce overall assembly costs.

FIG. 8 is a schematic representation of a laminated diamond microchannelheat sink, which has been attached to a strongback similar to that shownin FIG. 7. Heat sink 63′ is fabricated from a lamination of thinnerdiamond submounts 64 and 60 that are plated with gold adherent 101 and103 and attached to each other using AuSn solder 102. After beingjoined, microchannels 104 are cut using the laser in accordance with thesubject invention. The lamination approach allows thinner and thus lowercost diamond material to be fabricated into thicker heat sinks. It alsopermits the use of poorer quality diamond material with lower thermalconductivities at distances further removed from the heat source whereheat flux is reduced. Laminations consisting of three plies have beensuccessfully fabricated in the laboratory. In some embodiments, channels104 may not extend through all the plies as shown at 104′.

FIG. 9 is a view of a diamond microchannel heat sink 40, which has beenattached, via gold adherent and AuSn solder to a diamond strongback 66.Four power dissipating resistors have been gold adherent plated andsoldered to the strongback, and the diamond microchannel shown in FIG. 6has been soldered to the bottom of the strongback in accordance with thesubject invention.

Thus far, the fins have been shown to be formed in the top or bottomplanar surface of the diamond plate. This, however, is not a necessarylimitation of the subject invention as shown in FIG. 10 where diamondplate 70 has channels 72 cut in edge surface 74 so as not to interferewith top and bottom planar surfaces 76 and 78 thus rendering diamondplate 70 suitable for use as a window—i.e., a window between theenvironment and an infrared radiation detection subsystem in a missile.

In FIGS. 11-12, top surface 80 of diamond substrate 82 is renderedoptically reflective by a gold coating, for example, and channels 84 arecut in bottom surface 88 to form an optical device with integral coolingchannels.

In FIG. 2, no system packaging is shown. In contrast, in FIG. 13, thereare two diamond heat sinks 40 each with heat source 10 mounted theretoand this subassembly is mounted in package 90 which includes a vacuumbrazed aluminum manifold 92 which drives coolant 94 in the microchannelsof each heat sink.

Heat sink 40, FIGS. 5 and 6 was manufactured as follows. A CVD diamondblank approximately 1 mm thick and 125 mm diameter was grown using amicrowave assisted CVD diamond reactor at Raytheon Company's AdvancedMaterials Laboratory, Lexington, Mass., the assignee of the subjectapplication, and delivered to the Mechanical and Materials EngineeringLaboratory diamond cutting and polishing facility to be cut by a laserinto the desired rectangular shape and then ground to a near-opticalquality finish. The blanks were then cut into pieces measuring 18 mm by8 mm and placed in the laser cutting facility where a YAG laser wasprogrammed to transverse the diamond material is an X-Y plane withvarious material feed rates along the X-direction to provide the channelcuts at the approximate desired depth. At the completion of eachX-direction pass, the material was indexed approximately 50 um inches inthe Y-direction and the pass repeated in the opposite direction. Uponcompleting each channel cut, which required 3 passes, the material wasindexed 150 um in the Y-direction, and the channel cutting resumed.Channel depth, spacing and contours are controlled by virtue of theprogramming loaded into the piece drive controls, the laser pulse dutycycle, and the number of passes. This process resulted in producingchannels 50 with a depth of 800 um and a width of 150 um.

Accordingly, in accordance with the subject invention, the resistance toheat transfer from a heat source such as power amplifier chip orsemi-conductor laser-diode array to the environment is greatly improvedand numerous interfaces between the power dissipating chip and the heatsink eliminated by using a laser to cut microchannels in the diamondsubmount previously used as a lateral heat spreader to turn the diamondsubmount into a heat sink with fins and microchannels.

The diamond microchannel heat sink in accordance with the subjectinvention exhibits the capability to accommodate high heat flux levels(3,200 W/cm²)—an order of magnitude above current technology. As statedabove, the diamond heat sink of the subject invention performs twofunctions: heat spreading and heat dissipation. The life expectancy ofGaAs type chips is expected to increase by at least a factor of 2 perMil-HDBK-217F for a 25° C. reduction.

In full production runs, a diamond wafer up to 125 mm in diameter isgrown, cut to a convenient size and polished. Many heat sinks may bemanufactured at once by laser cutting the microchannels and then lasercutting the plurality of heat sinks from the wafer. Such heat sinks canbe used in conjunction with many different types of heat sources such aspower amplifiers, laser diode chips, integrated electronic devices(ASICs), optical devices, and the like.

Therefore, although specific features of the invention are shown in somedrawings and not in others, this is for convenience only as each featuremay be combined with any or all of the other features in accordance withthe invention. The words “including”, “comprising”, “having”, and “with”as used herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments.

Other embodiments will occur to those skilled in the art and are withinthe following claims:

1. A method of manufacturing a diamond heat sink, the method comprising:growing diamond to form a substrate; and using a laser to cut channelsin the substrate to form fins thereon.
 2. The method of claim 1 furtherincluding growing multiple diamond layers and securing the multiplediamond layers together to form a substrate with discrete layers beforethe channels are cut.
 3. The method of claim 2 in which cutting includescutting channels in all the layers.
 4. The method of claim 2 in whichcutting includes cutting channels in a subset of the layers.
 5. Themethod of claim 1 in which growing includes chemical-vapor-deposition ofdiamond.
 6. The method of claim 1 in which the channels are cut to be150 um or less in width to form microchannels.
 7. The method of claim 1in which the channels are cut to extend in one direction to formstraight fins.
 8. The method of claim 1 in which the channels are cut toextend in two different directions to form pin fins.
 9. The method ofclaim 1 further including the addition of a metalization layer to thesubstrate.
 10. The method of claim 1 in which the substrate is polishedbefore it is cut.
 11. A heat sink assembly comprising: a natural orpolycrystalline diamond substrate with fins formed thereon; and anatural or polycrystalline diamond support attached to the substrate.12. The heat sink of claim 11 in which the diamond ischemical-vapor-deposited diamond or diamond-like-carbon.
 13. The heatsink assembly of claim 11 in which the fins extend continuously alongthe substrate.
 14. The heat sink assembly of claim 11 in which the finsare pin fins.
 15. The heat sink assembly of claim 11 in which thesubstrate and the fins are monolithic.
 16. The heat sink assembly ofclaim 11 in which the substrate has a plurality of layers.
 17. The heatsink assembly of claim 16 in which the fins are cut in all of thelayers.
 18. The heat sink assembly of claim 16 in which the fins are cutin a subset of all the layers.
 19. The heat sink assembly of claim 11 inwhich the fins form microchannels in the substrate.
 20. The heat sinkassembly of claim 11 in which the substrate has opposing top and bottomplanar surfaces, and the fins are formed in one said planar surface. 21.The heat sink assembly of claim 11 further including metalization on thesupport and metalization on the substrate.
 22. The heat sink assembly ofclaim 11 further including a heat source mounted on the support.
 23. Theheat sink assembly of claim 22 further including metalization on theheat source.
 24. An optical device comprising: a reflective surface; anda heat sink adjacent the optical surface, the heat sink including anatural or polycrystalline diamond substrate with fins formed thereon.25. A window comprising: a natural or a polycrystalline diamondsubstrate with upper and lower surfaces; and fins formed in at least oneedge of the substrate.
 26. An integrated cooling system comprising: anintegrated electronic or optical device; and a natural orpolycrystalline diamond substrate mated on one surface with the deviceand including fins formed on the substrate for cooling the device.